The photonic electrons are produced from photon conversion of pi0, and also from other meson decays. These form the primary background to the non-photonic electrons we're interested in and need to be subtracted (as best we can) out of the inclusive electrons sample.

To identify photonic electrons, every inclusive electron track (those that satisfy the electron id cuts) is paired with another possible electron track. The partner track only needs to satisfy a loose dE/dx cut ( | NSigmaElectron | < 3 ) and a low p_T cut (p_T > 0.3 GeV).

Then we look at every pair and determine the cuts to define an electron as a photonic electron.

• DCA
• Opening angle
• Opening angle, phi
• Opening angle, theta
• Invariant mass

Since the photonic electrons are oppositely charged pairs, we use the same sign pairs to estimate the combinatorial background.

DCA

We want a pair of tracks that can be traced a point of conversion, so we put a cut on the DCA.

Cut: DCA < 1 cm

Opening angles

Because the photons are relativistic, we expect the resulting electron and positron from its conversion to be boosted in the forward direction and hence have a small opening angle between them.

Cuts:

Angle < 0.15 rad

Theta < 0.05 rad

Phi < 0.1 rad

Invariant mass

If the pair of electrons were the products of photon conversion, then we expect the invariant mass to be small. When we calculate the pair's invariant mass, we get a higher mass bump due to TPC resolution in the bend (xy) plane. We project the mass onto the rz plane to eliminate this effect.

Cut: pairInvMass < 0.1